A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, it is desirable to measure parameters of the patterned substrate, for example the overlay error between successive layers formed in or on it. This measurement may take place during the lithographic process, or separately from it, but is usually carried out using a separate metrology apparatus from the lithographic apparatus.
There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. One form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers direct a monochromatic radiation beam onto the substrate, the intensity of the scattered radiation being measured as a function of angle.
The scatterometer may be adapted to measure the overlay of targets in the form of two misaligned gratings or periodic structures by measuring asymmetry in the reflected spectrum and/or the detection configuration, the asymmetry being related to the extent of the overlay. In order to use smaller overlay targets, which may be positioned on the scribe lanes between products or on the product itself, it is known to use small measurement spots, that is the area of the incident radiation on the target. However, the use of such small measurement spots means that although the spots may fill small targets, averagely sized targets or large targets will be under-filled by the measurement spot such that for a grating imperfections in each grating line will affect the measured angle-resolved spectrum more strongly than in the case that all the grating lines contribute.
US 2002/0135783 discloses a method and apparatus for evaluating periodic structures formed on a sample in which a probe beam is continuously scanned over a wafer until sufficient data can be measured. However such an arrangement produces a lower signal to noise ratio than with a larger measurement spot.
U.S. Pat. No. 6,023,338 discloses a method of determining offset between adjacent layers of a semiconductor device in which a measurement spot scans across the gratings on a wafer.